Photoconducting powders and method of preparation



United States v Patent 7 PHOTOCONDUCTING POWDERS AND METHOD OF PREPARATION "Charles J. Busanovich, Princeton, and Soren M. Thomsen,

Pennington, N. 1., assignors to Radio Corporation of America, a corporation of Delaware This invention relates to improved photoconducting powders which are particularly useful in gap type and area type photocells and in other devices having bodies including photoconductive powders incorporated therein. The invention includes improved methods for preparing hotoconducting powders and improved devices utilizing the photoconducting powders of the invention.

A photoconductive device is one which displays a reduced resistance to electric current flow when irradiated, as, for example, with light. In its simplest form, a photo conductive device comprises a body of photoconductive material and a pair of electrodes attached thereto. With a voltage applied to the electrodes, the device passes an increased amount of electric current when there is an increase in the intensity of light irradiating the device.

Ideally, a photoconductive device is a perfect insulator when light to which it is sensitive is absent, and is a per feet conductor when a maximum intensity of light to which it is sensitive is present. Actually, a photoconduo tive device behaves as a high resistance conductor when light to which it is sensitive is absent, and behaves as a lower resistance conductor when irradiated with light to which the device is sensitive. p

The change in conduction produced by a unit variation of light intensity is referred to as the photosensitivity of the device. The measure of photosensitivity is in terms of photocurrent under standard conditions. The current passed by the device in darkness is referred to as the dark current; the current passed when the device is irradiated is referred to as the light current; and the diflference between the light-current and the dark current is referred to as the-photocurrent.

One type of photoconductve device comprises a single crystal of a photoconductive material and electrodes attached to the crystal. Such single crystal photocells exhibit large photocurrents and high ratios of light current to dark current. and consequently the total current passed by a single crystal is small. When greater currents are passed through the crystal, the crystal heats up and the photosensitivity of the crystal is reduced, either temporarily or permanently. Furthermore, photoconductive crystals are difiicult to grow and are fragile. Thus, the expense of manufacture and maintenance often prohibits the use of single crystal photocells.

Another type of photoconductive device comprises a body including finely-divided photoconducting powder particles and electrodes attached to said body. The body may include, for example, an unbonded powder or a powder mixed with a binder such as a synthetic resin. Such powder photocells often exhibit a broader band of spectral response than single crystal photocells. In addition, powder photocells may be made in any desired size, shape or current carrying capacity. However, powder photocells have had the disadvantage of relatively low photosensitivity when the device is irradiated-with light to which his sensitive.

However, the crystals are small in size.

The low photosensitivity and high resistance of powder cells is generally attributed to the large number of electrical barriers existing between the electrodes. The elec. tric current passing between the electrodes must travel through a chain of powder particles. The resistance due .to poor electric contact between adjacent particles is multiplied by the number of particles in the chain, partly or completely masking the photosensitivity of the volume of each particle by limiting the maximum amount of current that can be passed by each chain of particles and by heating the particles during the flow of electric current. i Powders which have been prepared as phosphors and which exhibit photoconductivity, include cadmium sulphide and cadmium selenide with activator proportions of copper or silver and prepared with or without a halide. Bodies of these phosphor powders exhibit very low photo sensitivity and therefore have been of little commercial value as photoconductive materials. Similarly, large single crystals of cadmium sulphide and cadmium selenide which exhibit high photosensitivity have been crushed to a powder. Bodies of these crushed powders exhibit practically no photosensitivity.

An object of the invention is to provide improved photoconducting powders.

Another object is to improve the photosensitivity of photoconducting powders, and bodies including photoconducting powders.

Another object is to provide photoconducting powder particles, the surfaces of said particles providing low resistance contact to one another in the presence of incident radiation in-the range between 4000 A. and 9000 A. A further object is to provide improved photoconducting powders and bodies including Photoconducting powders comprising particles in the size range between 0.1 and 1.7 mils and having a dark resistivity above 10.0 ohm cm. and a light resistivity below 10 ohm cm. in the presence of about 5 foot candles of light from an incandescent source.

Another object is to provide improved photoconductive devices comprising the improved powders and bodies of the invention.

A further object is to provide improved methods for preparing photoconducting powders.

Photoconducting powders according to the invention comprise particles of a host crystal selected from the group consisting of selenides, sulphides,- and sulpho-selenides of cadmium having incorporated therein activator proportions of a halide and activator proportions of a metal selected from the group consisting of copper and silver, said particles being adapted to make low resistance contact to one another in the presence of incident radiation in the range between 4000 A. and 9000 A. A photoconductive body according to the invention comprises a mass of the photoconducting powder of the invention with or without a binder. The devices according to the invention comprise a body of the photoconducting powder according to the invention and electrodes attached thereto.

The improved method for producing a photoconducting powder comprises recrystallizing a material selected from the group consisting of sulphides, selenides and sulpho-selenides of cadmium to a desired range of particle size, incorporating into said recrystallized material activator proportions of a halide and activator proportions of ametal selected from the group consisting of copper and silver. By carefully controllingthe firing process, the surfaces of the particles of said recrystallized material make low resistance electrical contact to one another in the presence of incident radiation in the range between 4000A. and 9000 A. By providing such low resistance electrical contact between particles the photo Patented Mar. 3, 1959' sensitivity of the particles is unmasked, facilitating the flow of photocurrents through a body of the powder.

The invention will be more fully described in the following detailed description when read in conjunction with the drawings wherein:

Figure lis a sectional. view of an apparatus for preparing the photocon-ducting powder according to one method of the invention,

Figure 2 is a series of curves illustrating the spectral sensitivities of some typical photoconducting powders of the invention,

Figure 3 is a perspective view of a photocell according to the invention.

Figure 4 is a linear scale curve illustrating the current-voltage characteristic of a typical photoconducting powder of the invention,

Figure 5 is the curve of Figure 4 with the current ordinate plotted on a logarithmic scale, and

Figure 6 isa sectional view of an apparatus for preparing the photoconducting powder according to another method of the invention.

Similar reference characters are used for similar elements throughout the drawings.

Example 1.-A preferred method for preparing the photoconducting powder according to the invention follows.

An intimate mixture of 100 grams of cadmium sulphide, 10 grams of cadmium chloride, 1 gram of ammonium chloride, 1.7 milliliters of 0.1 M copper chloride, and 250 milliliters of water is prepared. This mixture may be prepared in a blender such as is used for mixing powders with water. The yellow, viscous liquid is dried at about 150 C. for about hours.

Referring to Figure 1, the dried cake is then broken up into pea-size lumps and packed. into a 12 inch test tube 21 to a depth of about seven inches. The tube 21 is provided with a stopper 23 having an inlet tube 25 therethrough for the purpose of maintaining a. substantially stagnant atmosphere in the test tube 21 while maintaining atmospheric pressure through the subsequent firing steps. The test tube 21 filled with the dried mixture 27, is fired at about 600 C. for about minutes and the fired product is then removed from the test tube 21 and allowed to soak in water until it disintegrates. This ordinarily takes about 20 minutes. The product is washed on a fine, sintered, glass filter, dispersing the cake once or twice in water until thewashings contain less than 0.01 M cadmium chloride.

The product of the first firing is brown in color a relatively fine particle size. During the first firing, there is present in the charge about 10% cadmium chloride which is a solvent flux for cadmium sulphide.

completely dissolve in the cadmium chloride and are re crystallized into small crystals which are of the order of 0.3 mil in size and have copper and halide incorporated therein. At this stage, the product is photoconducting.

The washed product of the first firing is moistened with a solution containing equal parts of 0.1 M aqueous cadmium chloride and 1.0 M ammonium chloride. The excess solution is removed by suction. After drying, the powder is passed through a 325 mesh sieve and the tailingsv discarded.

Referring again to Figure l, the dry powder 27 is placed in a test tube 21 to a depth not greater than 4.5 inches and fired for about 20 minutes at 600 C. in a stagnant atmosphere. The fired mass 27 is removed from the furnace and permitted to cool. During this second firing, the powder sint'ers to a stick, which is'then grated through a 50 mesh sieve. During the second firing there is present only a controlled trace of superficial chloride. The gently sintered stick is easily broken up into a powder which exhibits a low dark resistivity and high dark current.

Referring again to Figure 1, about 0.2 gram of sulphur is placed in the bottom of a test tube 21 and the sieved brown powder from the second firing is placed on top of the sulphur to a depth of about 4.5 inches. The powder in the test tube 21 is fired at about 500 C. for about 10 minutes in a stagnant atmosphere and then while still third firing, a vacuum is applied to the powder by means of inlet tube 25 and the firing continued for about 10 minutes with the vacuum applied. The test tube 21 is removed from the furnace, cooled and the product passed through a 325 mesh screen.

During the third firing, the sulphur vaporizes and passes through the mass of brown powder. The product of the third firing exhibits a high dark resistivity, low dark current and extremely high photosensitivity. Typical measurements indicate the ratio of light current to dark current of about 10 and a high speed of response.

The cadmium sulphide photoconducting powder which is the product of the third firing is brown to nearly black in color, the color darkening with increases in, either the proportions of copper or increases of the first firing temperature. The average particle size varies according to the first firing temperature, being of the order of 0.3 mil for 600 C. and of the order of 0.7 mil for 650 C. Referring to Figure 2 the powders. exhibit a panchromatic absorption, although the spectral response is peaked in red and is practically nil in the blue region of the spectrum as indicated by cur /e31. The powder is non-luminescent, has a substantially uniform particle size and is free-flowing.

The method of Example 1 may be varied for example, cadmium selenides and cadmium. sulphoselenides may be substituted for cadmium sulphide. Cadmium sulphide is soluble in molten cadmium chloride at least to the extent of 30%. The presence of about 10% cadmium chloride in the mix during the first firing permits the growth of discrete uniformly sized crystals bonded by the .waterv soluble cadmium chloride. The

and of The small particles of cadmium sulphide partially or fired lump disintegrates readily in water. Thus, the content of cadmium chloride is not critical, its purpose being principally to provide a crystallizing medium for the cadmium'sulphide host crystal. Although cadmium chloride is preferred, any crystallizing medium for the host crystal which does not otherwise adversely effeet the product, may be used in place of cadmium chloride. Similar crystallizing media are used for other host crystals.

Ammonium chloride is introduced into the mix to (1). convert to cadmium chloride any cadmium oxide which may be present in the mix, and (2) to provide a firing atmosphere that prevents oxidation. An activator proportion of chloride is incorporated into the host crystal during the first firing. This amount is extremely small and may come from either the cadmium chloride or the ammonium chloride.

Copper is introduced into the mix in a proportion equivalent to parts per million of copper with respect to cadmium sulphide. The amount of copper is not critical; however, it is preferred to use between 50 and 300 parts per million of copper. In place of copper, other monovalent cations may be incorporated into the cadmium sulphide host crystal. For example, 200 parts per million of silver in place of copperp'roduces an orangecolored powder having an intermediate photosensitivity and a low rate of decay.

The firing temperature during the first firing is somewhat critical. The firing temperature should be above the melting point of cadmium chloride which is about 550 C. Below this temperature practically no crystal growth occurs and the copper does not diffuse into the host crystal. Higher temperatures during the first firing produces a powder which has a darker color, larger particle size, lower dark resistivity and higher dark current. The preferred temperature is the lowest temperature that insures prompt melting of the solvent material, produces a small particle size and a high dark resistivity in the final product. A temperature of about 600 C. is preferred.

The second firing sinters the powder into a stick and increases the conductivity and photosensitivity of the material. Small particles are probably sintered onto the surface of the larger ones, thus, reducing the number of particle-to-particle contacts. Again, a firing temperature of the order of 600 C. is preferred as the lowest temperature which insures the prompt melting of cadmium chloride.

During the third firing, the sulphur vapor which passes through the mass of photoconducting powder, reduces the dark current of the powder, presumably by diminishing the chloride. in the-powder to a value substantially equivalent to the amount of copper present in the product. At 500 C., the powder does not sinter and the photosensitivity of the powder is only slightly affected. At higher temperatures, the loss in photosensitivity is greater.

In each of the firing steps enough time should be allowed to bring the entire charge to the furnace temperature. For tubes about one inch in diameter, about 20 minutes is required. Longer firings up to one hour make no noticeable difierence in the powder. Similarly, the speed with which the product is cooled makes little or no difference in the final product.

Grinding the finished powder progressively reduces its photosensitivity, similar to the observations on single crystals. Grinding of an intermediate product is undesirable because it produces all particle sizes and shapes, and because grinding is inherently uncontrollable. It is best, therefore, to avoid grinding at any stage. The powder from the first firing is put through a 325 mesh sieve to eliminate the few lumps which may have formed in earlier steps in the process and also to establish an upper limit (1.7 mils) to particle size. After the second firing, the 50 mesh sieve is used to achieve a more uniform disintegration of the sintered stick and to avoid crushing. After the third firing the final product is passed through a 325 mesh to eliminate any aggregates over 1.7 mils. Less than 5% is lost at this stage. In passing material through a sieve no hard rubbing is used. a The electrical properties of the final product are influenced by the amount of chloride present during the second firing. With too much chloride, the final product has a high dark current; with too little chloride, the final product has a low sensitivity.

Another method, according to the invention, is to crystallize the cadmium sulphide from a molten solvent containing a halide, and then to leach out the solvent after cooling. Copper is then added difiused into the cadmium sulphide particles during the second firing. Approximately the same proportion of copper produces the same results as described in Example 1.

Recrystallizing the cadmium sulphide from a molten halide solvent without copper present produces an electrically-conducting cadmium sulphide powder as the product of the first firing. Its conductivity is equivalent to the conductivity of the highly illuminated photoconductive powder. Such a powder is useful in certain types of electr'oluminescent devices where it is desired to improve the electrical contact between a photoconductive layer and a layer of electroluminescent material. a Referring to FigureS, a photocell according to the invention comprises a pair of spaced electrodes 52 attached to a photoconducting body 54 comprising the phot'oconducting powder of the invention. A device used to test the photoconducting powders of the invention is prepared as follows. A glassplate 50 is provided with a strip of conducting material 200 mils wide and having a 20 mil gap running across and at right angles to the strip providing two electrodes 52.

'A mixture is prepared comprising a quantity of the photoconducting powder of the invention and an equal volume of a 1% solution of ethyl cellulose in amyl alcohol. A drop 54 of this mixture is placed on the gap and allowed to dry. When the drop 54 is dry, the photocell is ready for use. A voltage, usually 300 volts, unless otherwise indicated, is applied to the electrodes. Tables I and II summarize the terminology notation and conditions of measurement.

While the test photocell utilizes a photoconducting body 54 comprising a resin-bonded powder, an unbonded powder may also be used. Other resins may be'used as a binder, for example, a silicone resin.

TABLE I Terminology and notations TABLE II Standard conditions Gap width 20 mils.

Gap length 200 mils. Powder binder ethyl cellulose. Solvent amyl alcohol. Illumination l ft.-candle., Voltage 300 volts.

Table III gives the data for some typical measurements of powders prepared andtested according to the invention. Conducting CdS:Cl is the product of the first firing of a mixture without copper, as described above.

TABLE III Typical measurements Al FOUR STAGESDURING PREPARATION v n t, t in R s raw material 300 0.02 0. 02 0.02 0. 02 1 0 first firing 300 300 10 0.02 10 4 4 second firing 160 1,000 750 600 500 2 .3 third firing 300 700 10 1 0.001 10 70 COMPARISON MATERIALS, HIGH AND LOW CONDUCTIVITY conducting CdS: CL- 1,000 1,000 1,000 1.000 1 0 (MS (H23 fired)- 300 0.0002 0.0002 0. 0002 0.0002 1 0 A photoconductive device having a gap 20 by 200 mils can dissipate about 0.5 watt (about I ma. at 400 volts). Burn-out, usually occurs somewhat above this. Large area photocells have been prepared capable of passing electric currents of up to several amperes.

Figures 4 and 5 show the photocurrent as a function of voltage for a typical test photocell prepared with the photoconducting powders of the invention. A transition occurs at about volts. Above this, i is linear with voltage and below this log i is linear with voltage. This indicates the presence of barriers presumably at the points of contact between the particles, totaling about 150 volts for the series of barriers along 20 mil distance. The curve of Figure 4 fits the equation: V-150=i (0.2).

' The data of Table III indicates that, with the mixture of photoconducting powder of Example 1 and ethyl cellulose'applied across a 20 by 200 mil gap with 300 volts;applied, there is available about 4 volts per particle. About half of this, .or' 2 volts per. particle, is. a barrier due to the resistive; surface contact between particles. Assuming a single layer of powder across the gap, about 1,000 microamperes. current is, carried by 700 chains of particles. This is equivalent to about 1 microampere per chain :and per particle.

The powders of the invention have a dark resistivity above 10 ohm cm. and a light resistivity below 10 ohm cr'n. when, illuminated with about foot candles of light. Such a range provides the necessary properties for com mercial applications.

The voltage appliedto a photocell such as the one above described may be either alternating or direct current. If an alternating current is used, the i /i ratio decreases with increased frequency for two reasons: first, because the z decreases as a function of the time constant of the photoconductor, and second, because the i increases as a function of the capacitance, across the gap. The specific examples of operational characteristics given herein are in connection with direct current operation. Such examples are equally representative of low frequency alternating current operation.

Example 2.-Another photoconducting powder of the invention is prepared by the method described in Example 1 except that cadmium selenide is substituted for cadmium sulphide in the starting mixture. Thus, the starting mixture comprises 100 grams of cadmium selenide, grams of cadmium chloride, 1 gram of ammonium chloride, 1.7 milliliters of 0.1 M copper chloride, and 250 milliliters of water.

Example 3.--Referring toFigure 6, 50 grams of cadmium sulphide, 8- milliliters of 0.01 M copper nitrate solution and 0.006 grain of cadmium chloride is slurried inzwater, dried at about 250* C., placed in a test tube 43 and fired at' about .700 C. for minutes in an atmosphere offflowing' hydrogen. Upon cooling the powder is ready for use- Example 3 utilizes about .100 parts per million of copper with respect to cadmium sulphide. Amounts between 10 and 10,000 parts per million may be used in this method. In place of hydrogen, any gas which is inert to cadmium sulphide, such as nitrogen, helium or hydrogen sulphide may be, used. Firing times from 10 to 60 minutes at 900 C. may be used. Firing temperatures from 500 C. to 1200 C. may be used with a suitable firing time.

Example 4.-The same mixture as described in Ex: ample 3 is fired in an atmosphere of flowing nitrogen containing free bromine. by passing 50 milliliters per minute of nitrogen through bromine water. it is preferred to introduce the brominecontaining nitrogen into the powder mass so that it passes freely through the powder during firing.

Satisfactory powders are obtained by each of the methodsdescribcd in Examples 3 and 4. However, the method of Example; 1 is preferred because consistently uniform products are obtained.

While the phenomenon herein described is not clearly understood, the following theory is ofiered as an explanation merely to aid the teaching of the invention. There is no intent to limit the invention to the following theory. When cadmium sulphide crystals grow in molten cadmium chloride or in a vapor phase containing a chloride, some chloride is incorporated into the crystal. Each Cl ion that enters the lattice displaces one S" ion. -One free electron also enters the lattice to preserve electroneutrality. The free electrons probably come from an oxidation of the 5 ions to free sulphur. This reaction does not occur with pure cadmium sulphide.

When sulphur or hydrogen. sulphide is introduced during a firing operation in a sufficient preponderance over chloride, 8- probably replaces Clions-from the lattice.

A150, the excess, electrons previously introduced for charge compensation are removed to preserve electro- Such a mixture may be obtained neutrality. Thus, the method of the invention includes steps for introducing free electrons. as charge compensators and steps for removing, the free electrons.

If Cu' ions are present with Clions during crystal growth or are later added, electrical conductivity is greatly reduced. Each Cu ion probably substitutes for a Cd+ ion and each (llion probably substitutes for. an 8- ion in the crystal. Since each Cu ion. makes. the crystal deficient in one electron, the excess electron brought into the lattice by the Clion compensates for this deficiency, thus preserving electroneutrality in the lattice. Although copper is introduced into the. mix us. Go, it nevertheless enters the lattice as Ctr- Itis believed that'Cu+ is reduced during firing.

Thus, when equal amounts of Cl ions and Cu ions are present, electroneutrality is established, and the crystal is insulating in the darkness. When light irradiates the crystal, the charge-compensating electrons are easily excited and wander through the lattice imparting a high conductivity to the crystal. When there is an excess of Cl" ions over Cu+ ions, the excess charge-compensating electrons produce increased conductivity in the darkness.

By the method of the invention, a balance of copper and a halide is attained, such that a maximum insulating value is obtained in the darkness and a maximum number of photo-excitable electrons are provided in the volume of the crystal. The photosensitivity of the volume is usually masked due to the high sensitivity of the contact between the particles. Treatment of the particles ac? cording to the invention adapts the surface of each particle to make a low resistance contact with other particles with which it is in physical contact when light is applied thereto.

The powders of the invention may be utilized to prepare photoconductive devices and elements that are useful in meters, relays, picture converters, picture intensifiers, pickup devices, switches and so forth. The devices comprise a body including the photoconducting powder of the invention and an electrode attached to said body. The body may be any shape; however, it is preferred to prepare the body as a layer of material.

There have been described novel, photoconducting powders having unusually high photosensitivities. There has also been described novel methods for preparing the photoconducting powders of the invention and photoconducting bodies and photoconducting devices comprising the photoconducting powders of the invention.

What is claimed is:

l. A method of preparing a photoconducting powder comprising recrystallizing cadmium sulphide from molten cadmium chloride containing activator proportions of copper, as a copper salt, washing said recrystallized cadmium sulphide to remove water-soluble material including said cadmium chloride, coating said washed cadmium sulphide particles with a thin layer of a halide, firing said coated cadmium sulphide particles to diffuse said halide into said cadmium sulphide, retiring said fired cadmium sulphide particles in a sulphur-containing atmosphere until the dark conductivity of said fired CdS is reduced to a desired value.

2. A method for preparing a photoconducting powder comprising firing a mixture comprising parts byweight of cadmium sulphide, 10 parts by weight of cadmium chloride, 1 part by weight of ammonium chlo ride, and .0001 part by weight of copper as copper chloride to about 600 C. for about 20 minutes, washing said fired product to remove water soluble materials, refiring said washed product with a trace of cadmium chlorides at about 600 C. for about 20 minutes, and then refiring saidrefired. product in a sulphur containing atmosphere at about 500 C. for about 10 minutes.

3. A. method for preparing a photoconducting powder comprising firing a mixture comprising 100 parts byweight of cadmium selenide, 10. parts by weight. of cad:

mium chloride, 1 part by weight of ammonium chloride, and .0001 part by weight of copper as copper chloride to about 600 C. for about 20 minutes, washing said fired product to remove water soluble materials, refiring said washed product with a trace of cadmium chlorides at about 600 C. for about 20 minutes, and then refiring said refired product in a sulphur-containing atmosphere at about 500 C. for about 10 minutes.

4. A method for preparing a photoconducting powder comprising recrystallizing from a molten solvent a material selected from the group consisting of sulfides, selenides, and sulfo-selenides of cadmium, said molten solvent containing activator proportions of a cation selected from the group consisting of copper and silver, washing said recrystallized material to remove water-soluble constituents, diffusing a trace of halide into said washed material, and then firing said halide-diffused material in a sulfur-containing atmosphere.

5. A photoconducting powder prepared by the method of claim 4.

6. A method for preparing a photoconducting powder comprising recrystallizing from a molten halide a material selected from the group consisting of sulfides, selenides, and sulfo-selenides of cadmium, said molten halide containing activator proportions of copper as a salt thereof, washing said recrystallized material to remove water-soluble constituents, firing said washed material with a trace of a halide to difiuse said halide into said material, and then refiring said fired material in a 1% sulfur-containing atmosphere until the dark conductivity oi said material is reduced to a desired value.

7. A method for preparing a photoconducting powder comprising recrystallizing cadmium sulfide from a molten halide, said molten halide containing activator proportions of copper as a salt thereof, washing said recrystallized cadmium sulfide to remove water-soluble constituents, firing said washed material with a trace of a halide to diffuse said halide into said material, and then refiring said fired material in a sulfur-containing atmosphere until the dark conductivity of said material is reduced to a desired value.

8. A method for preparing a photoconducting powder comprising recrystallizing cadmium selenide from a molten halide, said molten halide containing activator proportions of copper as a salt thereof, washing said recrystallized cadmium selenide to remove water-soluble constituents, firing said washed material with a trace of a halide to diffuse said halide into said material, and then refiring said fired material in a sulfur-containing atmosphere until the dark conductivity of said material is reduced to a desired value.

References Cited in the file of this patent UNITED STATES PATENTS 2,651,700 Gans Sept. 8, 1953 2,765,385 Thomsen Oct. 2, 1956 

4. A METHOD FOR PREPARING A PHOTOCONDUCTING POWDER COMPRISING RECRYSTALLIZING FROM A MOLTEN SOLVENT A MATERIAL SELECTED FROM THE GROUP CONSISTING OF SULFIDES, SELENIDES, AND SULFO-SELENIDES OF CADMIUM, SAID MOLTEN SOLVENT CONTAINING ACTIVATOR PROPORTIONS OF A CATION SELECTED FROM THE GROUP CONSISTING OF COPPER AND SILVER, WASHING SAID RECRYSTALLIZED MATERIAL TO REMOVE WATER-SOLUBLE CONSTITUENTS, DIFFUSING A TRACE OF HALIDE INTO SAID WASHED MATERIAL, AND THEN FIRING SAID HALIDE-DIFFUSED MATERIAL IN A SULFUR-CONTAINING ATMOSPHERE. 